(1) Field of the Invention
The present invention relates to an image forming apparatus such as a digital copier, facsimile machine, laser printer and the like, in particular relating to an image forming apparatus which forms images by causing developer particles to jump to the recording medium.
(2) Description of the Prior Art
Among image forming apparatuses for outputting image data as a visual image on recording medium such as recording paper etc., one type is known which directly forms a toner image on the recording medium by making toner, the developer, jump onto the recording medium, as has been disclosed in Japanese Patent Application Laid-Open Hei 6 No.155,798. As shown in FIG. 1, the image forming apparatus includes an image forming unit 51 having a toner supplying section 52 and a printing section 53. In this apparatus, toner 71 is made to jump from toner supplying section 52 and adhere to a sheet of paper 55, the recording medium. During this, the jumping of toner 71 is controlled in accordance with the image data.
Toner supplying section 52 is composed of a toner reservoir 70 for holding toner 71 as developer particles which are negatively charged, and a toner support 72 which supports toner 71 on its peripheral surface by magnetic force whilst rotating in the direction of arrow E. Printing section 53 is composed of an opposing electrode 75 of a cylindrical shape and a control electrode 76 which is provided between opposing electrode 75 and toner support 72. Opposing electrode 75 rotates in the direction of arrow F so that paper 55 is conveyed between opposing electrode 75 and control electrode 76 in the direction of arrow G.
As shown in FIG. 2A, control electrode 76 has a plurality of gates 79 formed therein, each gate 79 having an annular electrode 77 formed around the edge thereof. As the voltage from a control power source 81 shown in FIG. 1 is selectively applied to these annular electrodes 77 in accordance with the image data, toner 71 supported on the peripheral surface of toner support 72 is made to jump toward opposing electrode 75 and pass through selective gates 79 hence being made to adhere to paper 55 which is placed between opposing electrode 75 and control electrode 76.
The image forming apparatus configured as above is one which directly forms the image on the surface of recording medium such as paper etc. Therefore, it is no longer necessary to use a developer medium such as a photoreceptor etc., which was used in conventional image forming apparatuses. Further, the operation for transferring the image from the developer medium to the paper can be omitted, thus making it possible to eliminate degradation of the image due to the existence of this operation. Moreover, the structure of the apparatus can be simplified needing fewer parts, thus making it possible to reduce the apparatus in size and cost.
However, in the above conventional image forming apparatus, since the peripheral velocity of toner support 72 and the conveying speed of paper 55 are equal, if an arbitrary gate 79n has been made to pass toner 71 therethrough and subsequently the adjoining gate 79n+1 is made to pass toner 71 therethrough as shown in FIG. 2B, a toner-free area will be produced on the peripheral surface of toner support 72 by the transfer of toner 71 through gate 79n. Because toner support 72 rotates during the time between the two events, part of this area overlaps the subsequent printing area designated at 72n+1 on the peripheral surface of toner support 72 that opposes gate 79n+1 as shown in FIG. 2C. As a result, area 72n+1 on the peripheral surface of toner support 72 might partially lack the toner 71, which may be needed for later transfer. Therefore, the amount of toner 71 transferred through gate 79n+1 becomes low resulting in insufficient dot density and dot diameter in the formed image, lowering the image contrast and degrading the reproduction of halftone. In color image forming apparatus, it becomes impossible to reproduce the desired colors. Moreover, image deficiency such as white strips and color voids may occur.
In order to avoid such degradation of the image, it is considered that the density of toner 71 on the peripheral surface of toner support 72 needs to be increased. However, there is a limit for the toner density on toner support 72, and it is impossible to obtain an adequate toner density under the present conditions. Further, in this case, the thickness of the toner layer on toner support 72 tends to become unstable and in some cases, it may become difficult to obtain the desired layer thickness, making it impossible to form an stable image.
Moreover, when the thickness of the toner layer on toner support 72 is enhanced to increase the density of toner 71, the distance between control electrode 76 and toner 71 becomes shortened. This means that toner 71 becomes more likely to adhere to control electrode 76 and becomes further unstable in its layer thickness. If toner 71 has adhered to control electrode 76, the potential created by the charge carried by toner 71 changes the potential of control electrode 76 resulting in an ineptness in controlling the potential used for image forming. This also causes obstruction or clogging in gates 79.
In order to avoid the above situation, if control electrode 76 is made more distant from toner support 72, the potentials to be applied to toner support 72, opposing electrode 75 and control electrode 76 need to be increased, resulting in increase in cost for the power source. In addition, the elevation of the potential applied to opposing electrode 75 requires a more thorough insulation and also the price for the high-voltage driver for switching the potential applied to control electrode 76 increases.